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KEY POINTS (continued)

in the apical membrane of the epithelial cells. K+ can also be secreted by the colon by a process that involves K+ channels in the apical membrane.

• Water absorption takes place through the cellular and paracellular pathways in response to local osmotic gradients. Throughout most of the small intestine, the absorbate (i.e., absorbed solution) is isotonic.

• Secretory cells are located in the crypts of the small and large intestines. Their basolateral membranes contain a furosemide inhibitable carrier-mediated process that results in the entry of one Na+ ion, one K+ ion, and two Cl" ions per cycle, energized by the Na+ gradient across that barrier.

• The apical membranes of intestinal secretory cells contain cystic fibrosis transmembrane conductance regulator (CFTR) channels and Ca2+-activated Cl" channels. Secretion can be stimulated by physiologic and/or pathophysiologic secretagogues that promote an increase in either cyclic adenosine monophosphate (cAMP) or Ca2+.

OVERVIEW OF THE DAILY TASK: INPUT VERSUS OUTPUT

In addition to efficiently digesting and absorbing the many grams of nutrients (i.e., carbohydrates, proteins, and fats) ingested daily, as described in Chapter 38, the intestinal tract faces a prodigious task in absorbing ions and water.

Normally, the average human ingests approximately 5-10 g of NaCl per day and consumes about 2 L of water. But approximately 7 L of water and 25 g of Na +

are added to this ingested load by salivary, gastric, biliary, pancreatic, and intestinal secretions (Fig. 1). These secretions represent approximately 20% of our total body water (see Chapter 1) and 15% of our total body Na+ content. Clearly, failure to reabsorb this impressive load would rapidly lead to serious dehydration and electrolyte imbalance.

Approximately, 7.5 L or 85% of the total load of water and Na+ presented to the small intestine are absorbed by that organ; this represents only about 40% of its maximum absorptive capacity. Approximately 1.5 L of

FIGURE 1 Approximate volumes of fluid entering the small intestine daily, absorbed by the small and large intestines and excreted in the feces. (From Sellin JH. The pathophysiology of diarrhea. In: Schultz SG, Andreoli TE, Brown AM, Fambrough DM, Welsh MJ, eds. Molecular biology of membrane transport disorders. New York: Plenum Press, 1996; 541-563.)

FIGURE 1 Approximate volumes of fluid entering the small intestine daily, absorbed by the small and large intestines and excreted in the feces. (From Sellin JH. The pathophysiology of diarrhea. In: Schultz SG, Andreoli TE, Brown AM, Fambrough DM, Welsh MJ, eds. Molecular biology of membrane transport disorders. New York: Plenum Press, 1996; 541-563.)

36. Intestinal Electrolyte and Water Transport fluid cross the ileocecal valve daily and enter the colon, and more than 95% of this load is absorbed. The maximum absorptive capacity of the large intestine is estimated to be 4-6 L per day. Thus, there is a large absorptive reserve capacity that serves to protect the individual from excessive losses of fluid and electrolytes when small intestine function is impaired. However, the colon is incapable of absorbing most nutrients (i.e., the products of protein, fat, and carbohydrate digestion), so any of these that escape absorption by the small intestine will be lost in the stool or broken down by colonic bacteria.

One of the products of bacterial catabolism of undigested and unabsorbed carbohydrates that pass through the ileocecal valve are short-chain free fatty acids such as acetate, propionate, and butyrate. Because these are readily absorbed by the large intestine, this organ is capable of salvaging some nutrients that escape from the small intestine. In herbivores, short-chain free fatty acids absorbed by the colon contribute significantly to the nutritional needs of the animal. In humans and other omnivores, the extent to which they contribute to the caloric intake is considerably less under normal conditions, but it can become significant if absorptive capacity of the small intestine is decreased.

The daily fecal excreta contains approximately 100 mL of water and 25-50 g of solids. The solids are for the most part bacteria (30%), undigested dietary fiber (30%), lipids (10-20%), and inorganic matter (10-20%). Although luminal contents that enter the colon from the ileum are relatively rich in Na+ and Cl", the stool is normally relatively rich in K+ and HCO3".

The remainder of this chapter focuses on how water and electrolytes are transported by the small and large intestines and how impairment of these normally highly efficient processes may result in life-threatening diarrheas (derived from the Greek, "to run through'').

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